During the manufacture of lead-acid storage batteries, it is often the practice to fill the assembled storage batteries with an acid solution for initially forming the battery plates. In a mass production facility, it is desirable to fill the batteries with acid in as short a time span as possible. After the filling and formation steps, the batteries may either be prepared for shipment with the acid retained, or the batteries may be spun "dry" as described in U.S. Pat. No. 3,738,490 issued June 12, 1973 to Tigerman and assigned to the assignee of the present invention.
One commonly used method of filling storage batteries with acid is the vacuum process. Basically, a vacuum manifold is placed over the battery filling ports and a vacuum created in the individual cell compartments. Acid is then drawn into the battery cells until a desired fill level is attained. A disadvantage of the vacuum fill process is that the cells do not always fill at a uniform rate whereby acid from the earlier filling cells is often drawn out and replaced with fresh acid while the slower filling cells receive acid. Since initial contact of the acid with the battery plates results in some dilution of the acid due to combination of sulfate ions with lead in the plates and release of water, the faster filling cells which receive fresh acid will have a higher specific gravity than the slower filling cells. Another disadvantage of the vacuum process arises in connection with the use of thin-walled battery containers such as described in U.S. Pat. No. 3,388,007 issued June 11, 1968 to Fiandt. In thin-walled batteries, the vacuum in the battery cells causes the end walls to be drawn in slightly thereby decreasing the end cell volume temporarily. Upon removal of the vacuum, the end walls will return to their original position and the cell volume increases whereby the acid level will drop.